The present invention relates to a magnetic toner used in image forming methods, such as electrophotography, electrostatic recording, magnetic recording and toner jetting; a process for production of the magnetic toner; and an image forming method, an image forming apparatus and a process cartridge using the magnetic toner.
Hitherto, many proposals have been made regarding a magnetic toner (i.e., a magnetically susceptible toner) and an image forming method using the toner.
U.S. Pat. No. 3,909,258 has proposed a developing method using a magnetic toner, having electroconductivity. According to the proposal, an electroconductive magnetic toner is supplied onto a cylindrical electroconductive sleeve enclosing a magnet at an inside thereof and is caused to contact an electrostatic latent image for development. In this instance, at the developing position, an electroconductive path is formed of the toner particles between a recording member surface and the sleeve surface, charges are guided to the toner particles from the sleeve via the electroconductive path, and the charged toner particles are attached onto an image part of an electrostatic image due to a Coulomb force acting between the image part and the toner particles, thereby to effect a development. The developing method using such an electroconductive magnetic toner is an excellent method capable of obviating the problems accompanying the conventional two-component developing method, but on the other hand, involves a problem that it becomes difficult to effect the transfer of a developed image from the recording member to a final supporting material, such as plain paper, because the toner is electro-conductive.
As a developing method using a high-resistivity magnetic toner allowing electrostatic transfer, a developing method using dielectric polarization of toner particles is known. However, such a method involves essential problems of a slow developing speed or inability of obtaining a sufficiently high image density.
Other known developing methods using a high-resistivity insulating magnetic toner includes a method wherein toner particles are triboelectrically charged through friction between individual toner particles, friction between a sleeve and toner particles, etc. This method is accompanied with a problem that the toner particles are liable to have an insufficient triboelectric charge leading to image defects due to charging failure because of a low opportunity of contact between the toner particles and the friction member and the magnetic toner particles used contain much magnetic powder exposed to the toner particle surfaces.
Japanese Laid-Open Patent Application (JP-A) 55-18656 and others have proposed a jumping developing method, wherein a magnetic toner is applied as a very thin coating layer, then triboelectrically charged and then brought to very proximity to an electrostatic image to develop the electrostatic image. This method is excellent in that the magnetic toner is applied in a very thin layer on the sleeve to increase the opportunity of contact between the sleeve and the toner, thereby allowing a sufficient triboelectric charge. However, such a developing method using an insulating magnetic toner is accompanied with uncertain factors inherent to the use of an insulating magnetic toner. Such uncertain factors are caused by exposure of a portion of magnetic fine powder mixed and dispersed in a substantial amount in the insulating magnetic toner, and as a result, several performances, such as developing performance and durability, required of a magnetic toner, are changed or deteriorated.
It is considered that the above-mentioned problem encountered in the case of using a conventional magnetic toner containing magnetic powder has been principally caused by exposure of the magnetic powder to the magnetic toner surface. More specifically, if magnetic powder having a relatively low resistivity is exposed to the surface of magnetic toner particles principally composed of a resin having a larger resistivity, toner performance lowering are caused, such as a lowering in toner chargeability, lowering in toner flowability, and a lowering in image density or occurrence of a density irregularity called sleeve ghost caused by liberation of the magnetic powder due to friction between individual toner particles and between toner particles and the regulating member during a long term of use. Hitherto, proposals have been made regarding magnetic iron oxide contained in magnetic toners, but have left problems yet to be solved.
For example, JP-A 62-279352 has proposed a magnetic toner containing silicon-containing magnetic iron oxide. In the magnetic iron oxide, silicon (element) is intentionally incorporated at an inner part of magnetic iron oxide particles, but the flowability of a magnetic toner containing the magnetic iron oxide has still left a room for improvement.
Japanese Patent Publication (JP-B) 3-9045 has proposed to control the shape of magnetic iron oxide particles into a spherical one by adding a silicate salt. As a result of the use of a silicate salt for particle shape control, the magnetic iron oxide particles contain much silicon inside thereof and have little silicon at the surfaces, thereby having a smooth surface, so that the resultant toner is caused to have somewhat improved flowability but the adhesion between the magnetic iron oxide particles and the binder resin constituting the magnetic toner is insufficient.
JP-A 61-34070 has proposed a process for producing triiron tetroxide characterized by addition of a hydrosilicate solution during oxidation to triiron tetroxide. The triiron tetroxide obtained through this process retains Si at proximity to its surface, but the Si is present in a layer proximate to the surface, so that the surface thereof is weak against a mechanical impact such as abrasion.
On the other hand, a toner is generally produced through the pulverization process, wherein toner ingredients such a a binder resin, a colorant, etc., are melt-kneaded for uniform dispersion, pulverized and classified to recover toner particles of a desired particle size. According to this process, however, the range of material selection is restricted if toner particle size reduction is intended. For example, it is necessary that the colorant-dispersed resin is sufficiently fragile and can be finely pulverized by an economically feasible apparatus. As a result of providing a fragile colorant-dispersed resin from this requirement, an actual high-speed pulverization of the colorant-dispersed resin is liable to result in particles of a broad particle size range, particularly including a relatively large proportion of fine powder fraction (excessively pulverized particles). Moreover, a toner of such a highly fragile material is liable to be further fine pulverization or powder formation during its use as a developer in a copying machine, etc.
Further, according to toner production by the pulverization process, it is difficult to completely uniformly disperse solid particles, such as magnetic powder or colorant into a resin, and a lower degree of dispersion is liable to result in increased fog and a lower image density. Further, the pulverization process essentially and inevitably results in exposure of magnetic iron oxide particles to the toner particle surfaces, thus leaving problems regarding toner flowability and charging stability in a severe environment.
Thus, the pulverization process essentially poses a limit in production of small-size toner particles required for high resolution and high-quality images, as it is accompanied with inevitable problems regarding uniform chargeability and flowability of the toner.
On the other hand, as the toner particle size is reduced, the particle size of the magnetic material used therefor is necessarily reduced correspondingly. For example, as for magnetite which is a magnetic material having a wide applicability and also functioning as a colorant, a higher coloring power is given at a smaller particle size and a smaller particle size is considered more advantageous from the viewpoint of probability for distribution of even amounts to individual toner particles in the case of smaller particle size toner production. However magnetite generally has a tendency of assuming a high residual magnetization at an increased surface area accompanying particle size reduction. Accordingly, in case where magnetite of smaller particle size exhibiting a higher coloring power is used, the magnetite is liable to cause magnetic agglomeration during toner production, thus leaving problems in developing performance in some cases. Moreover, the residual magnetization of the resultant toner particle is increased, so that the toner particles are liable to exhibit a lower flowability also due to magnetic agglomeration or a lower developing performance due to an increased magnetic constraint force exerted from the sleeve in the magnetic mono-component developing method. Moreover, during the continued use for a long period, a portion of the toner exhibiting a relatively low developing performance is gradually accumulated without being consumed for development, various problems, such as image density lowering occur. In this way, in order to provide a magnetic toner of smaller particle size with excellent performances, it becomes an important factor to uniformly disperse fine particle size magnetite of controlled magnetic properties in the toner.
As a proposal noting magnetic properties of a toner, JP-B 7-60273 has proposed a small-particle size toner obtained by classification into a specific particle size distribution and having residual magnetization of 1-5 emu/g (Am2/kg) prepared through the pulverization process. Further, Japanese Patent No. 2662410 has disclosed a pulverization toner having a residual magnetization of 2.7-5.5 emu/g and comprising a binder resin having a molecular weight distribution showing at least two peaks. The toners disclosed in these publications are however pulverization toners, and are therefore accompanied with difficulty in suppressing the exposure of the magnetic powder to the toner particle surfaces, so that they are accompanied with problems in dispersibility of the magnetic powder, toner flowability, charging stability in a severe environment, a lower circularity and transferability. Further, these publications include only Examples wherein a magnetic blade exerting less load on the toner is used as a toner layer thickness-regulating member in the image forming apparatus, so that these publications do not clarify at all how the toner residual magnetization affects the image quality in the case of using a toner layer thickness regulating member exerting a mechanical load on the toner, such as an elastic blade abutted against a toner-carrying member, for providing an improved toner chargeability.
In order to overcome the problems of the toner produced by the pulverization process and for complying with recent requirement for improved properties of the toner as mentioned above, the production of a toner through a suspension polymerization process has been proposed. A toner produced by suspension polymerization (hereinafter sometimes called xe2x80x9cpolymerization tonerxe2x80x9d) is advantageous for complying with higher image qualities, because of easiness for production of smaller toner particles, and production of spherical toner particles. However, if a polymerization toner contains magnetic powder, the flowability and the chargeability thereof are liable to be remarkably lowered. This is because magnetic powder is generally hydrophilic and is therefore liable to be present at the toner surface. In order to solve the problem, the surface property modification of magnetic powder becomes important.
As for surface treatment of magnetic powder for improved dispersion thereof in a polymerization toner, many proposals have been made. For example, JP-A 59-200254, JP-A 59-200256, JP-A 59-200257 and JP-A 59-224102 have proposed treatment of magnetic powder with various silane coupling agents, and JP-A 63-250660 and JP-A 10-239897 have disclosed treatment of silicon-containing magnetic powder with silane coupling agents. These treatments provide a somewhat improved dispersibility in the toner but are accompanied with a problem that it is difficult to uniformly hydrophobize magnetic powder surfaces, so that it is difficult to obviate the coalescence of magnetic powder particles and the occurrence of untreated magnetic powder particles, thus being insufficient to improve the dispersibility in the toner to a satisfactory level.
Incidentally, JP-A 10-20548 has disclosed a process for producing a polymerization toner by using a non-aromatic organic peroxide having a molecular weight of at most 250 as a polymerization initiator. According to the publication, it is possible to produce a toner which contains little polymerization initiator decomposition products or residual monomer and has little odor. However, the publication describes carbon as the colorant and does not clarify any regarding the effect in the case of using magnetic powder. Further, the amount of residual monomer provided as a result is still substantial, so that a further improvement is necessary. Further, in the process disclosed in the publication, the suspension liquid after the suspension polymerization is immediately subjected to the addition of an acid for acid washing of the toner particles without a prior filtration of the suspension liquid, so that a carboxylic acid as a polymerization initiator decomposition product is not dissolved in and removed together with the waste water but is allowed to remain in the toner particle in an amount substantially identical to that produced during the polymerization. As a result, the product toner is still accompanied with problems regarding not only odor at the time of heating but also fixability and chargeability according to our study.
JP-A 9-43904 has disclosed a process for producing a polymerization toner containing hydrophobized magnetic powder by using a peroxide polymerization initiator of bis(t-butylperoxy)hexane. The publication however does not disclose how the hydrophobization of magnetic powder was performed. The publication discloses a process wherein core particles are first produced by polymerization in the presence of an azo polymerization initiator and then the shell is formed by polymerization in the presence of the above-mentioned peroxide polymerization initiator. As a result, the publication does not clarify the effects in the case where toner particles are formed by polymerization of a polymerizable mixture including magnetic powder, styrene monomer and a peroxide polymerization initiator. In the disclosed process, only 46 wt. parts of magnetic powder is added per 100 wt. parts of the binder resin to produce core particles which are then coated with a shell resin, so that the magnetic polymer is presumably substantially completely enclosed at an inner portion in the toner particles. The thus-produced toner is used for providing a two-component developer.
Further, JP-B 4-73442 has disclosed a process wherein a resin for a toner is suspension-polymerized in the presence of partially saponified polyvinyl alcohol as a dispersing agent, followed by addition of an alkali metal hydroxide into the polymerization system, heating and filtration, to remove acidic impurities originated from the starting materials or by-produced during polymerization. However, no description is made regarding the production of a polymerization toner. Thus, the publication does not clarify at all what effects are attained when the alkali treatment is applied to the production of a polymerization toner containing magnetic powder.
In recent years, the printer utilizing the electrophotography includes an LED printer and an LBP printer which principally comply with the demand on the market and for which higher resolutions of 400, 600 and 1200 dpi are being required compared with conventional levels of 240-300 dpi. Accordingly, the developing scheme therefor is also required to have a higher resolution. Also in the copying apparatus, higher performances are required, and a principal demand is directed to a digital image forming technique as a trend. The digital image formation principally involves the use of a laser for forming electrostatic images for which a higher resolution is intended. Thus, similarly as in the printer, a developing scheme of a higher resolution and a higher definition is demanded. For complying with such demands, JP-A 1-112253 and JP-A 2-284158 have proposed toners of smaller particle sizes. However, the above-mentioned various problems have not been fully solved as yet.
As for developers for developing electrostatic images, there have been known a two-component developer comprising a carrier and a toner, and a mono-component developer (inclusive of a magnetic toner and a non-magnetic toner) requiring no carrier. The toner is charged principally by friction between the carrier and the toner in the two-component developer system, and principally by friction between the toner and a charge-imparting member in the mono-component developer system. Further, regardless of the toner is for the two-component developer or the mono-component developer, it has been widely practiced to add inorganic fine powder as an external additive to toner particles in order provide the toner with an improved flowability, an improved chargeability, etc.
For example, JP-A 5-66608 and JP-A 4-9860 disclose hydrophobized inorganic fine powder or inorganic fine powder hydrophobized and then treated with silicone oil. Further, JP-A 61-249059, JP-A 4-264453 and JP-A 5-346682 disclosed to add hydrophobized inorganic fine powder and silicone oil-treated inorganic fine powder in combination.
Further, many proposals have been made regarding addition of electroconductive fine powder as an external additive. For example, carbon black as electroconductive fine powder is widely known as an external additive to be attached to or fixed on toner particles for the purpose of, e.g., imparting electroconductivity to the toner, or suppressing excessive charge of the toner to provide a uniform triboelectric charge distribution. Further, JP-A 57-151952, JP-A 59-168458 and JP-A 60-69660 have disclosed to externally add electroconductive fine powder of tin oxide, zinc oxide and titanium oxide, respectively, to high-resistivity toner particles. JP-A 56-142540 has proposed a toner provided with both developing performance and transferability by adding electroconductive magnetic particles, such as iron oxide, iron powder or ferrite, to high-resistivity magnetic toner particles so as to promote charge induction to the magnetic toner. Further, JP-A 61-275864, JP-A 62-258472, JP-A 61-141452 and JP-A 02-120865 have disclosed the addition of graphite, magnetite, polypyrrole electroconductive fine powder and polyaniline electroconductive fine powder to the respective toners. Further, the addition of various species of electroconductive fine powder to the toner is known.
Hitherto, image forming methods, such as electrophotography, electrostatic recording, magnetic recording, and toner jetting have been known. In the electrophotography, for example, an electrical latent image is formed on a latent image-bearing member which is generally a photosensitive member comprising a photoconductor material by various means, the electrostatic image is developed with a toner to form a visible toner image, and the toner image is, after being transferred onto a recording medium, such as paper, as desired, followed by fixing of the toner image onto the recording medium under application of heat, pressure or heat and pressure to form a fixed image.
In the conventional image forming methods, the residual portion of the toner remaining on the image-bearing member after the transfer is generally recovered by various means into a waste vessel in a cleaning step, and the above-mentioned steps are repeated for a subsequent image forming cycle.
The toner recovery or cleaning step has been conventionally performed by using, e.g., a cleaning blade, a cleaning fur brush, a cleaning roller, etc. According to any of these methods, the transfer residual toner is mechanically scraped off or collected by damming into a waste toner vessel. The system including such a cleaning step has been generally accompanied with a difficulty that the life of the latent image-bearing member is shortened due to abrasion caused by abutting of the cleaning member against the latent image-bearing member. The provision of the cleaning device results in an increase in apparatus size and has provided an obstacle against apparatus compactization. From the viewpoints of resource economization, reduction of waste materials and effective utilization of toner, it has been desired to develop an image forming system which is free from waste toner and exhibits excellent fixability and anti-offset property.
In contrast thereto, a so-called development and simultaneous cleaning system (developing-cleaning system) or cleanerless system has been proposed as a system free from generation of waste toner. Such a system has been developed principally for obviating image defects, such as positive memory and negative memory due to residual toner. This system has not been satisfactory for various recording media which are expected to receive transferred toner images in view of wide application of electrophotography in recent years.
Cleanerless systems have been disclosed in, e.g., JP-A 59-133573, JP-A 62-203182, JP-A 63-133179, JP-A 64-20587, JP-A 2-302772, JP-A 5-2289, JP-A 5-53482 and JP-A 5-61383. These systems have not been described with respect to desirable image forming methods or toner compositions.
As for the developing step of developing a latent image with a toner, various methods have been known. For example, as methods for visualizing electrostatic latent images, the cascade developing method, the pressure developing method and the magnetic brush developing method using a two-component developer comprising a carrier and a toner, are known. There are also practiced the non-contact mono-component developing method of causing a toner to jump onto an image-bearing member from a toner-carrying member disposed in no contact with the image-bearing member, the magnetic mono-component developing method of causing a magnetic toner onto a photosensitive member from a rotating sleeve enclosing magnetic poles at an inside thereof and an electric field between the photosensitive member and the sleeve, and the contact mono-component developing method of transferring a toner under an electric field between an image-bearing member and a toner-carrying member abutted against the image-bearing member.
Among such various developing methods, as a developing method suitably applicable to a system essentially free from a cleaning device, a cleanerless system or a development and simultaneous cleaning system, it has been considered essential to rub the electrostatic latent image-bearing member surface with a toner and a toner-carrying member, so that contact developing methods wherein the toner or developer is caused to contact the latent image-bearing member have been principally considered. This is because the mode of rubbing the latent image-bearing member with the toner or developer has been considered advantageous for recovery of the transfer residual toner particles by developing means. However, such a development and simultaneous cleaning system or a cleanerless system is liable to cause toner deterioration, and the deterioration or wearing of the toner-carrying member surface or photosensitive member surface, so that a sufficient solution has not been given to the durability problem. Accordingly, a simultaneous development and cleaning system according to a non-contact developing scheme is desired.
On the other hand, as image forming methods applied to electrophotographic apparatus and electrostatic recording apparatus, various methods are also known as methods of forming latent images on image bearing members, such as an electrophotographic photosensitive member and an electrostatic recording dielectric member. In the electrophotography, for example, it is a general practice to uniformly charge a photosensitive member comprising a photoconductor as a latent image-bearing member in a desired polarity and at desired potential, and then subject the photosensitive member to imagewise pattern exposure to form an electrical latent image.
Hitherto, a corona charger (or corona discharger) has been generally used as a charging device for uniformly charging (including a case for charge removal) a latent image-baring member to desired polarity and potential.
A corona charger is a non-contact-type charging device comprising a discharge electrode such as a wire electrode and a shield electrode surrounding the discharge electrode while leaving a discharge opening, and the device is disposed in no contact with an image-bearing member as a member to be charged so that the discharge opening is directed to the image-bearing member for a prescribed charging operation wherein a high voltage is applied between the discharge electrode and the shield electrode to cause a discharge current (corona shower), to which the image-bearing member surface is exposed to be charged to a prescribed potential.
In recent years, a contact charging device has been proposed and commercialized as a charging device for a member to be charged such as a latent image-bearing member because of advantages, such as low ozone-generating characteristic and a lower power consumption, than the corona charging device.
A contact charging device is a device comprising an electroconductive charging member (which may also be called a contact charging member or a contact charger) in the form of a roller (charging roller), a fur brush, a magnetic brush or a blade, disposed in contact with a member-to-be-charged, such as an image-bearing member, so that the contact charging member is supplied with a prescribed charging bias voltage to charge the member-to-be-charged to prescribed polarity and potential.
The charging mechanism (or principle) during the contact charging may include (1) discharge (charging) mechanism and (2) direct injection charging mechanism, and may be classified depending on which of these mechanism is predominant.
This is a mechanism wherein a member is charged by a discharge phenomenon occurring at a minute gap between the member and a contact charging member. As a certain discharge threshold is present, it is necessary to apply to the contact charging member a voltage which is larger than a prescribed potential to be provided to the member-to-be-charged. Some discharge product occurs wile the amount thereof is remarkably less than in a corona charger, and active ions, such as ozone, occur though the amount thereof is small.
This is a mechanism wherein a member surface is charged with a charge which is directly injected into the member from a contact charging member. This mechanism may also be called direct charging, injection charging or charge-injection charging. More specifically, a charging member of a medium resistivity is caused to contact a member-to-be-charged to directly inject charges to the member-to-be-charged basically without relying on a discharge phenomenon. Accordingly, a member can be charged to a potential corresponding to an applied voltage to the charging member even if the applied voltage is below a discharge threshold. This mechanism is not accompanied with occurrence of active ions, such as ozone, so that difficulties caused by discharge products can be obviated. However, based on the direct injection charging mechanism, the charging performance is affected by the contactivity of the contact charging member onto the member-to-be-charged. Accordingly, it is preferred that the charging member is provided with a relative moving speed difference from the member-to-be-charged so as to provide a more frequent contact and more dense points of contact with the member-to-be-charged.
As a contact charging device, a roller charging scheme using an electroconductive roller as a contact charging member is preferred because of the stability of charging performance and is widely used. During the contact charging according to the conventional roller charging scheme, the above-mentioned discharge charging mechanism (1) is predominant.
A charging roller has been formed of a conductive or medium-resistivity rubber or foam material optionally disposed in lamination to provide desired characteristics. Such a charging roller is provided with elasticity so as to ensure a certain contact with a member-to-be-charged, thus causing a large frictional resistance. The charging roller is moved following the movement of the member-to-be-charged or with a small speed difference with the latter. Accordingly, even if the direct injection charging is intended, the lowering in charging performance, and charging irregularities due to insufficient contact, contact irregularity due to the roller shape and attachment onto the member-to-be-charged, are liable to be caused.
FIG. 7 is a graph illustrating examples of charging efficiencies for charging photosensitive members by several contact charging members. The abscissa represents a bias voltage applied to the contact charging member, and the ordinate represents a resultant charged potential provided to the photosensitive member. The charging performance in the case of roller charging is represented by a line A. Thus, the surface potential of the photosensitive member starts to increase at an applied voltage exceeding a discharge threshold of ca. xe2x88x92500 volts. Accordingly, in order to charge the photosensitive member to a charged potential of xe2x88x92500 volts, for example, it is a general practice to apply a DC voltage of xe2x88x921000 volts, or a DC voltage of xe2x88x92500 volts in superposition of an AC voltage at a peak-to-peak voltage of, e.g., 1200 volts, so as to keep a potential difference exceeding the discharge threshold, thereby causing the charged photosensitive member potential to be converged to a prescribed charged potential.
To describe based on a specific example, in a case where a charging roller is abutted against an OPC photosensitive member having a 25 xcexcm-thick photosensitive layer, the surface potential of the photosensitive member starts to increase in response to an applied voltage of ca. 640 volts or higher and thereafter increases linearly at a slope of 1. The threshold voltage may be defined as a discharge inclination voltage Vth. Thus, in order to obtain a photosensitive member surface potential Vd required for electrophotography, it is necessary to apply a DC voltage of Vd+Vth exceeding the required potential to the charging roller. Such a charging scheme of applying only a DC voltage to a contact charging member may be termed a xe2x80x9cDC charging schemexe2x80x9d. In the DC charging scheme, however, it has been difficult to charge the photosensitive member to a desired potential, since the resistivity of the contact charging member is liable to change in response to a change in environmental condition, and because of a change in Vth due to a surface layer thickness change caused by abrasion of the photosensitive member.
For this reason, in order to achieve a more uniform charging, it has been proposed to adopt an xe2x80x9cAC charging schemexe2x80x9d wherein a voltage formed by superposing a DC voltage corresponding to a desired Vd with an AC voltage having a peak-to-peak voltage in excess of 2xc3x97Vth is applied to a contact charging member as described in JP-A 63-149669. According to this scheme, the charged potential of the photosensitive member is converged to Vd which is a central value of the superposed AC voltage due to the potential smoothing effect of the AC voltage, whereby the charged potential is not affected by the environmental change. In the above-described contact charging scheme, the charging mechanism essentially relies on discharge from the contact charging member to the photosensitive member, so that a voltage exceeding a desired photosensitive member surface potential has to be applied to the contact charging member and a small amount of ozone is generated.
Further, in the AC-charging scheme for uniform charging, ozone generation is liable to be promoted, a vibration noise (AC charging noise) between the contact charging member and the photosensitive member due to AC voltage electric field is liable to caused, and the photosensitive member surface is liable to be deteriorated due to the discharge, thus posing a new problem.
Fur brush charging is a charging scheme, wherein a member (fur brush charger) comprising a brush of electroconductive fiber is used as a contact charging member, and the conductive fiber brush in contact with the photosensitive member is supplied with a prescribed charging bias voltage to charge the photosensitive member surface to prescribed polarity and potential. In the fur brush charging scheme, the above-mentioned discharge charging mechanism may be predominant.
As the fur brush chargers, a fixed-type charger and a roller-type charger have been commercialized. The fixed-type charger is formed by bonding a pile of medium-resistivity fiber planted to or woven together with a substrate to an electrode. The roller-type charger is formed by winding such a pile about a core metal. A fiber density of ca. 100/mm2 can be relatively easily obtained, but even at such a high fiber density, the contact characteristic is insufficient for realizing sufficiently uniform charging according to the direct injection charging. In order to effect a sufficiently uniform charging according to the direct injection charging, it is necessary to provide a large speed difference between the fur brush charger and the photosensitive member, and this is not practically feasible.
An example of the charging performance according to the fur brush charging scheme under DC voltage application is represented by a line B in FIG. 7. Accordingly, in the cases of fur brush charging using any of the fixed-type charger and the roller-type charger, a high charging bias voltage is applied to cause a discharge phenomenon to effect the charging.
In contrast to the above-mentioned charging schemes, in a magnetic brush scheme, a charging member (magnet brush charger) obtained by constraining electroconductive magnetic particles in the form of a magnetic brush under a magnetic field exerted by a magnet roll is used as a contact charging member, and the magnetic brush in contact with a photosensitive member is supplied with a prescribed charging bias voltage to charge the photosensitive member surface to prescribed polarity and potential. In the magnetic brush charging scheme, the above-mentioned direct injection charging scheme (2) is predominant. Uniform direct injection charging becomes possible, e.g., by using magnetic particles of 5-50 xcexcm in particle size and providing a sufficient speed difference with the photosensitive member.
An example of the charging performance according to the magnetic brush scheme under DC voltage application is represented by a line C in FIG. 3, thus allowing a charged potential almost proportional to the applied bias voltage. The magnetic brush charging scheme is however accompanied with difficulties that the device structure is liable to be complicated, and the magnetic particles constituting the magnetic brush are liable to be liberated from the magnetic brush to be attached to the photosensitive member.
Further, regarding the contact charging scheme and the contact transfer scheme, there is disclosed a method wherein an electroconductive elastic roller is abutted against an image-bearing member and is supplied with a voltage to uniformly charge the image-bearing member surface, followed by exposure and development to form a toner image, another electroconductive roller is abutted against the image-bearing member, and a transfer material is passed therebetween to transfer the toner image on the transfer material, followed by a fixing step to obtain a copy image (JP-A 63-149669 and JP-A 2-123385).
The contact charging scheme or the contact transfer scheme, unlike the corona discharge scheme, is accompanied with problems. More specifically, in the contact transfer step, the transfer member is abutted against the image-bearing member via a transfer material, so that the toner image is pressed between the image-bearing member and the transfer material by a pressing force exerted by the transfer member, thus being liable to cause a local transfer failure called xe2x80x9ctransfer (hollow) dropoutxe2x80x9d. In addition, in response to demand for high-resolution and high-definition images in recent years, there is a tendency of using small-particle size toners. As the toner particle size becomes smaller, compared with a Coulomb force acting on the toner particles in the transfer step, the forces acting for attaching the toner particles onto the image-bearing member (such as an image force and a van der Waals force) become relatively larger, to increase the transfer-residual toner.
On the other hand, in the contact charging step, the charging member is pressed against the image-bearing member surface, so that the transfer-residual toner is also pressed against the image-bearing member by the charging member, whereby the image-bearing member is liable to cause surface abrasion or wearing, and further toner melt-sticking is liable to occur at the abraded part of the image-bearing member as the nuclei. This is liable to be more pronounced as the transfer-residual toner is increased in amount.
The abrasion and toner melt-sticking on the image-bearing member result in serious defects in latent image formation on the image-bearing member. More specifically, the abraded part of the image-bearing member causes a primary charging failure to result in black spots in a halftone image, and the toner melt-sticking causes an exposure failure to result in white spots in a halftone image. Further, these surface defects result in poorer toner transfer. As a result, in combination with the above-mentioned transfer failure due to the contact transfer, the image defects can be synergistically promoted.
The abrasion and transfer failure on the image-bearing member is liable to be pronounced in the case of using a developer comprising indefinite-shaped toner particles. This is presumably because such an indefinite shaped toner is liable to scrape the image-bearing member surface in addition to its inherent lower transferability due to the shape.
The abrasion problem is promoted especially when a magnetic developer comprising toner particle surfaces at which the magnetic powder is exposed. This is readily understood since the exposed magnetic powder is directly pressed against the photosensitive member.
Further, in case where the transfer-residual toner is increased, it becomes difficult to retain a sufficient contact between the contact charging member and the photosensitive member to result in a lower chargeability, so that in the reversal development system, fog, i.e., toner transfer onto non-image parts, is liable to occur. This phenomenon becomes more noticeable in a low-humidity environment wherein the resistivities of the members are liable to increase.
In view of also such environmental factors, in order to realize an image forming method satisfactorily employing the contact charging scheme and the contact transfer scheme, it is desired to develop a magnetic toner (developer) which shows a high transferability and is free from the abrasion and toner melt-sticking on the image-bearing member.
Now, the application of such a contact charging scheme to a development and simultaneous cleaning method or a cleanerless image forming method as described, is considered.
The development and simultaneous cleaning method or the cleanerless image forming method does not use a cleaning member, so that the transfer residual toner particles remaining on the photosensitive member are caused to contact the contact charging system wherein the discharge charging mechanism is predominant. If an insulating toner is attached to or mixed into the contact charging member, the charging performance of the charging member is liable to be lowered.
In the charging scheme wherein the discharge charging mechanism is predominant, the lowering in charging performance is caused remarkably from a time when the toner layer attached to the contact charging member surface provides a level of resistance obstructing a discharge voltage. On the other hand, in the charging scheme wherein the direct injection charging mechanism is predominant, the lowering in charging performance is caused as a lowering in chargeability of the member-to-be-charged due to a lowering in opportunity of contact between the contact charging member surface and the member-to-be-charged due to the attachment or mixing of the transfer residual toner particles into the contact charging member. The lowering in uniform chargeability of the photosensitive member (member-to-be-charged) results in a lowering in contrast and uniformity of latent image after imagewise exposure, and a lowering in image density and increased fog in the resultant images.
Further, in the development and simultaneous cleaning method or the cleanerless image forming method, it is important to control the charging polarity and charge of the transfer residual toner particles on the photosensitive member and stably recover the transfer residual toner particles in the developing step, thereby preventing the recovered toner from obstructing the developing performance. For this purpose, the control of the charging polarity and the charge of the transfer residual toner particles are effected by the charging member.
This is more specifically described with respect to an ordinary laser beam printer as an example. In the case of a reversal development system using a charging member supplied with a negative voltage, a photosensitive member having a negative chargeability and a negatively charged toner, the toner image is transferred onto a recording medium in the transfer step by means of a transfer member applying a positive voltage. In this case, the transfer residual toner particles are caused to have various charges ranging from a positive polarity to a negative polarity depending on the properties (thickness, resistivity, dielectric constant, etc.) of the recording medium and the image area thereon. However, even if the transfer residual toner is caused to have a positive charge in the transfer step, the charge thereof can be uniformized to a negative polarity by the negatively charged charging member for negatively charging the photosensitive member.
As a result, in the case of a reversal development scheme, the negatively charged residual toner particles are allowed to remain on the light-part potential where the toner is to be attached, and some irregularly charged toner attached to the dark-part potential is attracted to the toner carrying member due to a developing electric field relationship during the reversal development so that the transfer residual toner at the dark-part potential is not allowed to remain thereat but can be recovered. Thus, by controlling the charging polarity of the transfer residual toner simultaneously with charging of the photosensitive member by means of the charging member, the development and simultaneous cleaning or cleanerless image forming method can be realized.
However, if the transfer residual toner particles are attached to or mixed to the contact charging member in an amount exceeding the toner charge polarity-controlling capacity of the contact charging member, the charging polarity of the transfer residual toner particles cannot be uniformized so that it becomes difficult to recover the toner particles in the developing step. Further, even if the transfer residual toner particles are recovered by a mechanical force of rubbing, they adversely affect the triboelectric chargeability of the toner on the toner-carrying member if the charge of the recovered transfer residual toner particles has not been uniformized.
Thus, in the development and simultaneous cleaning or cleanerless image forming method, the continuous image-forming performance and resultant image quality are closely associated with the charge-controllability and attachment-mixing characteristic of the transfer residual toner particles at the time of passing by the charging member.
Further, JP-A 3-103878 discloses to apply powder on a surface of a contact charging member contacting the member-to-be-charged so as to prevent charging irregularity and stabilize the uniform charging performance. This system however adopts an organization of moving a contact charging member (charging roller) following the movement of the member-to-be-charged (photosensitive member) wherein the charging principle generally relies on the discharge charging mechanism simultaneously as in the above-mentioned cases of using a charging roller while the amount of ozone adduct has been remarkably reduced than in the case of using a corona charger, such as scorotron. Particularly, as an AC-superposed DC voltage is used for accomplishing a stable charging uniformity, the amount of ozone adducts is increased thereby. As a result, in the case of a continuous use of the apparatus for a long period, the defect of image flow due to the ozone products is liable to occur. Further, in case where the above organization is adopted in the cleanerless image forming apparatus, the attachment of the powder onto the charging member is obstructed by mixing with transfer-residual toner particles, thus reducing the uniform charging effect.
Further, JP-A 5-150539 has disclosed an image forming method using a contact charging scheme wherein a developer comprising at least toner particles and electroconductive particles having an average particle size smaller than that of the toner particles is used, in order to prevent the charging obstruction due to accumulation and attachment onto the charging member surface of toner particles and silica fine particles which have not been fully removed by the action of a cleaning blade on continuation of image formation for a long period. The contact charging or proximity charging scheme used in the proposal is one relying on the discharge charging mechanism and not based on the direct injection charging mechanism so that the above problem accompanying the discharge mechanism accrues. Further, in case where the above organization is applied to a cleanerless image forming apparatus, larger amounts of electroconductive particles and toner particles are caused to pass through the charging step and have to be recovered in the developing step. No consideration on these matters or influence of such particles when such particles are recovered on the developing performance of the developer has been paid in the proposal. Further, in a case where a contact charging scheme relying on the direct injection charging scheme is adopted, the electroconductive fine particles are not supplied in a sufficient quantity to the contact charging member, so that the charging failure is liable to occur due to the influence of the transfer residual toner particles.
Further, in the proximity charging scheme, it is difficult to uniformly charge the photosensitive member in the presence of large amounts of electroconductive fine particles and transfer residual toner particles, thus failing to achieve the effect of removing the pattern of transfer residual toner particles. As a result, the transfer residual toner particles interrupt the imagewise exposure pattern light to cause a toner particle pattern ghost. Further, in the case of instantaneous power failure or paper clogging during image formation, the interior of the image forming apparatus can be remarkably soiled by the developer.
In order to improve the charge control performance when the transfer residual toner particles are passed by the charging member in the development and simultaneous cleaning method, JP-A 11-15206 has proposed to use a toner comprising toner particles containing specific carbon black and a specific azo iron compound in mixture with inorganic fine powder. Further, it has been also proposed to use a toner having a specified shape factor and an improved transferability to reduce the amount of transfer residual toner particles, thereby improving the performance of the development and simultaneous cleaning image forming method. This image forming method however relies on a contact charging scheme based on the discharge charging scheme and not on the direct injection charging scheme, so that the system is not free from the above-mentioned problems involved in the discharge charging mechanism. Further, these proposals may be effective for suppressing the charging performance of the contact charging member due to transfer residual toner particles but cannot be expected to positively enhance the charging performance.
Further, among commercially available electrophotographic printers, there is a type of development and simultaneous cleaning image forming apparatus including a roller member abutted against the photosensitive member at a position between the transfer step and the charging step so as to supplement or control the performance of recovering transfer residual toner particles in the development step. Such an image forming apparatus may exhibit a good development and simultaneous cleaning performance and remarkably reduce the waste toner amount, but liable to result in an increased production cost and a difficulty against the size reduction.
JP-A 10-307456 has disclosed an image forming apparatus adapted to a development and simultaneous cleaning image forming method based on a direct injection charging mechanism and using a developer comprising toner particles and electroconductive charging promoter particles having particle sizes smaller than xc2xd of the toner particle size. According to this proposal, it becomes possible to provide a development and simultaneous cleaning image forming apparatus which is free from generation of discharge product, can remarkably reduce the amount of waste toner and is advantageous for producing inexpensively a small size apparatus. By using the apparatus, it is possible to provide good images free from defects accompanying charging failure, and interruption or scattering of imagewise exposure light. However, a further improvement is desired.
Further, JP-A 10-307421 has disclosed an image forming apparatus adapted to a development and simultaneous cleaning method, based on the direct injection charging mechanism and using a developer containing electroconductive particles having sizes in a range of {fraction (1/50)}-xc2xd of the toner particle size so as to improve the transfer performance.
JP-A 10-307455 discloses the use of electroconductive fine particles having a particle size of 10 nm-50 xcexcm so as to reduce the particle size to below one pixel size and obtain a better charging uniformity.
JP-A 10-307457 describes the use of electroconductive particles of at most about 5 xcexcm, preferably 20 nm-5 xcexcm, so as to bring a part of charging failure to a visually less recognizable state in view of visual characteristic of human eyes.
JP-A 10-307458 describes the use of electro-conductive fine powder having a particle size smaller than the toner particle size so as to prevent the obstruction of toner development and the leakage of the developing bias voltage via the electroconductive fine powder, thereby removing image defects. It is also disclosed that by setting the particle size of the electroconductive fine powder to be larger than 0.1 xcexcm, the interruption of exposure light by the electroconductive fine powder embedded at the surface of the image-bearing member is prevented to realize excellent image formation by a development and simultaneous cleaning method based on the direct injection charging scheme. However, a further improvement is desired.
JP-A 10-307456 has disclosed a development and simultaneous cleaning image forming apparatus capable of forming without causing charging failure or interruption of imagewise exposure light, wherein electroconductive fine powder is externally added to a toner so that the electroconductive powder is attached to the image-bearing member during the developing step and allowed to remain on the image-bearing member even after the transfer step to be present at a part of contact between a flexible contact charging member and the image-bearing member.
These proposals however have left a room for further improvement regarding the stability of performance during repetitive use for a long period and performance in the case of using smaller size toner particles in order to provide an enhanced resolution.
A generic object of the present invention is to solve the above-mentioned problems of the prior art.
A more specific object of the present invention is to provide a magnetic toner free from generating unpleasant odor at the time of printing and showing a quick chargeability even in a relatively high temperature/high humidity environment.
Another object of the present invention is to provide a magnetic toner less liable to cause toner melt-sticking onto a toner layer thickness-regulating member or a photosensitive member and capable of maintaining high-quality images even in continuous printing on a large number of sheets.
A further object of the present invention is to provide a process for producing the above-mentioned magnetic toner.
Another object of the present invention is to provide an image forming method using the magnetic toner, free from generating discharge products and capable of remarkably reducing the waste toner.
Another object of the present invention is to provide an image forming method adopting a developing-cleaning step (i.e., a development and simultaneous cleaning step or a cleanerless system) and yet capable of stably obtaining good chargeability.
A further object of the present invention is to provide an image forming method adopting a developing-cleaning step and yet capable of exhibiting a good transferability and good performance in recovery of transfer-residual toner.
A further object of the present invention is to provide an image forming apparatus adopting a developing-cleaning system advantageous for production of an inexpensive compact apparatus and yet capable of providing good images free from charging failure even in a long period of repetitive use.
A still further object of the present invention is to provide an image forming apparatus and a process cartridge therefor capable of stably providing good images even in the case of small-size toner particles in order to realize a higher resolution.
According to the present invention, there is provided a magnetic toner, comprising: magnetic toner particles each comprising at least a binder resin and magnetic toner, and inorganic fine powder; wherein the magnetic toner has an average circularity of at least 0.970,
the magnetic toner has a magnetization of 10 -50 Am2/kg at a magnetic field of 79.6 kA/m,
the magnetic powder comprises at least magnetic iron oxide,
the magnetic toner particles retain carbon in an amount of A and iron in an amount of B at surfaces thereof as measured by X-ray photoelectron spectroscopy, satisfying: B/A less than 0.001,
the binder resin comprises a resin formed by polymerization of a monomer comprising at least styrene monomer,
the magnetic toner has a residual styrene monomer content of less than 300 ppm, and the magnetic toner contains at least 50% by number of toner particles satisfying a relationship of
D/Cxe2x89xa60.02,
wherein C represents a volume-average particle size of the magnetic toner, and D represents a minimum distance between a magnetic toner particle and the magnetic toner contained in the magnetic toner particles.
The present invention further provides a process for producing the magnetic toner, and an image forming method, an image forming apparatus and a process cartridge using the above-mentioned magnetic toner.